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<tr class="t-nv"><td colspan="5"> <a href="requires.html" title="cpp/language/requires"> Requires expression</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span> </td></tr>
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</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Allows customizing class <span class="t-rev-inl t-since-cxx14"><span>and variable</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span> templates for a given category of template arguments.
</p>
<h3><span class="mw-headline" id="Syntax">Syntax</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>template</b></code> <code><b>&lt;</b></code> <span class="t-spar">parameter-list</span> <code><b>&gt;</b></code> <span class="t-spar">class-key</span> <span class="t-spar">class-head-name</span> <code><b>&lt;</b></code> <span class="t-spar">argument-list</span> <code><b>&gt;</b></code> <span class="t-spar">declaration</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>template</b></code> <code><b>&lt;</b></code> <span class="t-spar">parameter-list</span> <code><b>&gt;</b></code> <span class="t-spar">decl-specifier-seq</span> <span class="t-spar">declarator</span> <code><b>&lt;</b></code> <span class="t-spar">argument-list</span> <code><b>&gt;</b></code> <span class="t-spar">initializer</span><span class="t-mark">(optional)</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx14">(since C++14)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>where <span class="t-spar">class-head-name</span> identifies the name of a previously declared <a href="class_template.html" title="cpp/language/class template">class template</a><span class="t-rev-inl t-since-cxx14"><span> and <span class="t-spar">declarator</span> identifies the name of a previously declared <a href="variable_template.html" title="cpp/language/variable template">variable template</a> </span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span>. This declaration must be in the same <a href="namespace.html" title="cpp/language/namespace">namespace</a> or, for <a href="member_template.html" title="cpp/language/member template">member templates</a>, class scope as the primary template definition which it specializes .
</p><p>For example,
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T1, <span class="kw1">class</span> T2, <span class="kw4">int</span> I<span class="sy1">&gt;</span>
<span class="kw1">class</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>             <span class="co1">// primary template</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw4">int</span> I<span class="sy1">&gt;</span>
<span class="kw1">class</span> A<span class="sy1">&lt;</span>T, T<span class="sy2">*</span>, I<span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>   <span class="co1">// #1: partial specialization where T2 is a pointer to T1</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> T2, <span class="kw4">int</span> I<span class="sy1">&gt;</span>
<span class="kw1">class</span> A<span class="sy1">&lt;</span>T<span class="sy2">*</span>, T2, I<span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>  <span class="co1">// #2: partial specialization where T1 is a pointer</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw1">class</span> A<span class="sy1">&lt;</span><span class="kw4">int</span>, T<span class="sy2">*</span>, <span class="nu0">5</span><span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// #3: partial specialization where</span>
                        <span class="co1">//     T1 is int, I is 5, and T2 is a pointer</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X, <span class="kw1">class</span> T, <span class="kw4">int</span> I<span class="sy1">&gt;</span>
<span class="kw1">class</span> A<span class="sy1">&lt;</span>X, T<span class="sy2">*</span>, I<span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>   <span class="co1">// #4: partial specialization where T2 is a pointer</span></pre></div></div>
<p>Examples of partial specializations in the standard library include <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../memory/unique_ptr.html"><span class="kw734">std::<span class="me2">unique_ptr</span></span></a></span></span>, which has a partial specialization for array types.
</p>
<h3><span class="mw-headline" id="The_argument_list">The argument list</span></h3>
<p>The following restrictions apply to the <span class="t-spar">argument-list</span> of a partial template specialization:
</p>
<div class="t-li1"><span class="t-li">1)</span> The argument list cannot be identical to the non-specialized argument list (it must specialize something):
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T1, <span class="kw1">class</span> T2, <span class="kw4">int</span> I<span class="sy1">&gt;</span> <span class="kw1">class</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>        <span class="co1">// primary template</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X, <span class="kw1">class</span> Y, <span class="kw4">int</span> N<span class="sy1">&gt;</span> <span class="kw1">class</span> B<span class="sy1">&lt;</span>X, Y, N<span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// error</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td> 
<p>Moreover, the specialization has to be more specialized than the primary template
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> N, <span class="kw1">typename</span> T1, <span class="kw1">typename</span>... <span class="me1">Ts</span><span class="sy1">&gt;</span> <span class="kw1">struct</span> B<span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span>... <span class="me1">Ts</span><span class="sy1">&gt;</span> <span class="kw1">struct</span> B<span class="sy1">&lt;</span><span class="nu0">0</span>, Ts...<span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// Error: not more specialized</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table></div>
<div class="t-li1"><span class="t-li">2)</span> Default arguments cannot appear in the argument list</div>
<div class="t-li1"><span class="t-li">3)</span> If any argument is a pack expansion, it must be the last argument in the list</div>
<div class="t-li1"><span class="t-li">4)</span> Non-type argument expressions can use template parameters as long as the parameter appears at least once outside a <a href="template_argument_deduction.html#Non-deduced_contexts" title="cpp/language/template argument deduction">non-deduced context</a> (note that only clang supports this feature currently):
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> I, <span class="kw4">int</span> J<span class="sy1">&gt;</span> <span class="kw1">struct</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> I<span class="sy1">&gt;</span> <span class="kw1">struct</span> A<span class="sy1">&lt;</span>I<span class="sy2">+</span><span class="nu0">5</span>, I<span class="sy2">*</span><span class="nu0">2</span><span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>  <span class="co1">// error, I is not deducible</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> I, <span class="kw4">int</span> J, <span class="kw4">int</span> K<span class="sy1">&gt;</span> <span class="kw1">struct</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> I<span class="sy1">&gt;</span> <span class="kw1">struct</span> B<span class="sy1">&lt;</span>I, I<span class="sy2">*</span><span class="nu0">2</span>, <span class="nu0">2</span><span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK: first parameter is deducible</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">5)</span> Non-type template argument cannot specialize a template parameter whose type depends on a parameter of the specialization:
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, T t<span class="sy1">&gt;</span> <span class="kw1">struct</span> C <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// primary template</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> C<span class="sy1">&lt;</span>T, <span class="nu0">1</span><span class="sy1">&gt;</span><span class="sy4">;</span>   <span class="co1">// error: type of the argument 1 is T,</span>
                                    <span class="co1">// which depends on the parameter T</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> X, <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>array_ptr<span class="br0">)</span><span class="br0">[</span>X<span class="br0">]</span><span class="sy1">&gt;</span> <span class="kw1">class</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// primary template</span>
<span class="kw4">int</span> array<span class="br0">[</span><span class="nu0">5</span><span class="br0">]</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> X<span class="sy1">&gt;</span> <span class="kw1">class</span> B<span class="sy1">&lt;</span>X, <span class="sy3">&amp;</span>array<span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// error: type of the argument &amp;array is</span>
                                       <span class="co1">// int(*)[X], which depends on the parameter X</span></pre></div></div></div>
<h3><span class="mw-headline" id="Name_lookup">Name lookup</span></h3>
<p>Partial template specializations are not found by name lookup. Only if the primary template is found by name lookup, its partial specializations are considered. In particular, a using declaration that makes a primary template visible, makes partial specializations visible as well:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> N
<span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T1, <span class="kw1">class</span> T2<span class="sy1">&gt;</span> <span class="kw1">class</span> Z <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// primary template</span>
<span class="br0">}</span>
<span class="kw1">using</span> N<span class="sy4">::</span><span class="me2">Z</span><span class="sy4">;</span> <span class="co1">// refers to the primary template</span>
 
<span class="kw1">namespace</span> N
<span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> Z<span class="sy1">&lt;</span>T, T<span class="sy2">*</span><span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>     <span class="co1">// partial specialization</span>
<span class="br0">}</span>
Z<span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy2">*</span><span class="sy1">&gt;</span> z<span class="sy4">;</span> <span class="co1">// name lookup finds N::Z (the primary template),</span>
               <span class="co1">// the partial specialization with T = int is then used</span></pre></div></div>
<h3><span class="mw-headline" id="Partial_ordering">Partial ordering</span></h3>
<p>When a class <span class="t-rev-inl t-since-cxx14"><span>or variable</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span> template is instantiated, and there are partial specializations available, the compiler has to decide if the primary template is going to be used or one of its partial specializations. 
</p>
<div class="t-li1"><span class="t-li">1)</span> If only one specialization matches the template arguments, that specialization is used</div>
<div class="t-li1"><span class="t-li">2)</span> If more than one specialization matches, partial order rules are used to determine which specialization is more specialized. The most specialized specialization is used, if it is unique (if it is not unique, the program cannot be compiled)</div>
<div class="t-li1"><span class="t-li">3)</span> If no specializations match, the primary template is used
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// given the template A as defined above</span>
A<span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">int</span>, <span class="nu0">1</span><span class="sy1">&gt;</span> a1<span class="sy4">;</span>   <span class="co1">// no specializations match, uses primary template</span>
A<span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy2">*</span>, <span class="nu0">1</span><span class="sy1">&gt;</span> a2<span class="sy4">;</span>  <span class="co1">// uses partial specialization #1 (T=int, I=1)</span>
A<span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">char</span><span class="sy2">*</span>, <span class="nu0">5</span><span class="sy1">&gt;</span> a3<span class="sy4">;</span> <span class="co1">// uses partial specialization #3, (T=char)</span>
A<span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">char</span><span class="sy2">*</span>, <span class="nu0">1</span><span class="sy1">&gt;</span> a4<span class="sy4">;</span> <span class="co1">// uses partial specialization #4, (X=int, T=char, I=1)</span>
A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy2">*</span>, <span class="kw4">int</span><span class="sy2">*</span>, <span class="nu0">2</span><span class="sy1">&gt;</span> a5<span class="sy4">;</span> <span class="co1">// error: matches #2 (T=int, T2=int*, I=2)</span>
                     <span class="co1">//        matches #4 (X=int*, T=int, I=2)</span>
                     <span class="co1">// neither one is more specialized than the other</span></pre></div></div></div>
<p>Informally "A is more specialized than B" means "A accepts a subset of the types that B accepts".
</p><p>Formally, to establish more-specialized-than relationship between partial specializations, each is first converted to a fictitious function template as follows:
</p>
<ul><li> the first function template has the same template parameters as the first partial specialization and has just one function parameter, whose type is a class template specialization with all the template arguments from the first partial specialization
</li><li> the second function template has the same template parameters as the second partial specialization and has just one function parameter whose type is a class template specialization with all the template arguments from the second partial specialization.
</li></ul>
<p>The function templates are then ranked as if for <a href="function_template.html#Function_template_overloading" title="cpp/language/function template">function template overloading</a>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> I, <span class="kw4">int</span> J, <span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> X <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// primary template</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> I, <span class="kw4">int</span> J<span class="sy1">&gt;</span>          <span class="kw1">struct</span> X<span class="sy1">&lt;</span>I, J, <span class="kw4">int</span><span class="sy1">&gt;</span>
<span class="br0">{</span>
    <span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> s <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// partial specialization #1</span>
<span class="co1">// fictitious function template for #1 is</span>
<span class="co1">// template&lt;int I, int J&gt; void f(X&lt;I, J, int&gt;); #A</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw4">int</span> I<span class="sy1">&gt;</span>                 <span class="kw1">struct</span> X<span class="sy1">&lt;</span>I, I, <span class="kw4">int</span><span class="sy1">&gt;</span>
<span class="br0">{</span>
    <span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> s <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// partial specialization #2</span>
<span class="co1">// fictitious function template for #2 is </span>
<span class="co1">// template&lt;int I&gt;        void f(X&lt;I, I, int&gt;); #B</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    X<span class="sy1">&lt;</span><span class="nu0">2</span>, <span class="nu0">2</span>, <span class="kw4">int</span><span class="sy1">&gt;</span> x<span class="sy4">;</span> <span class="co1">// both #1 and #2 match</span>
<span class="co1">// partial ordering for function templates:</span>
<span class="co1">// #A from #B: void(X&lt;I,J,int&gt;) from void(X&lt;U1, U1, int&gt;): deduction OK</span>
<span class="co1">// #B from #A: void(X&lt;I,I,int&gt;) from void(X&lt;U1, U2, int&gt;): deduction fails</span>
<span class="co1">// #B is more specialized</span>
<span class="co1">// #2 is the specialization that is instantiated</span>
    <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> x.<span class="me1">s</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// prints 2</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="mw-headline" id="Members_of_partial_specializations">Members of partial specializations</span></h3>
<p>The template parameter list and the template argument list of a member of a partial specialization must match the parameter list and the argument list of the partial specialization. 
</p><p>Just like with members of primary templates, they only need to be defined if used in the program.
</p><p>Members of partial specializations are not related to the members of the primary template.
</p><p>Explicit (full) specialization of a member of a partial specialization is declared the same way as an explicit specialization of the primary template.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw4">int</span> I<span class="sy1">&gt;</span> <span class="co1">// primary template</span>
<span class="kw1">struct</span> A
<span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// member declaration</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw4">int</span> I<span class="sy1">&gt;</span>
<span class="kw4">void</span> A<span class="sy1">&lt;</span>T, I<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>      <span class="co1">// primary template member definition</span>
 
<span class="co1">// partial specialization</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> A<span class="sy1">&lt;</span>T, <span class="nu0">2</span><span class="sy1">&gt;</span>
<span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="kw4">void</span> h<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// member of partial specialization</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw4">void</span> A<span class="sy1">&lt;</span>T, <span class="nu0">2</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">g</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
 
<span class="co1">// explicit (full) specialization</span>
<span class="co1">// of a member of partial specialization</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
<span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">char</span>, <span class="nu0">2</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">h</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    A<span class="sy1">&lt;</span><span class="kw4">char</span>, <span class="nu0">0</span><span class="sy1">&gt;</span> a0<span class="sy4">;</span>
    A<span class="sy1">&lt;</span><span class="kw4">char</span>, <span class="nu0">2</span><span class="sy1">&gt;</span> a2<span class="sy4">;</span>
    a0.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, uses primary template’s member definition</span>
    a2.<span class="me1">g</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, uses partial specialization's member definition</span>
    a2.<span class="me1">h</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, uses fully-specialized definition of</span>
            <span class="co1">// the member of a partial specialization</span>
    a2.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error: no definition of f() in the partial</span>
            <span class="co1">// specialization A&lt;T,2&gt; (the primary template is not used)</span>
<span class="br0">}</span></pre></div></div>
<p>If a primary template is a member of another class template, its partial specializations are members of the enclosing class template. If the enclosing template is instantiated, the declaration of each member partial specialization is instantiated as well (the same way declarations, but not definitions, of all other members of a template are instantiated).
</p><p>If the primary member template is explicitly (fully) specialized for a given (implicit) specialization of the enclosing class template, the partial specializations of the member
template are ignored for this specialization of the enclosing class template.
</p><p>If a partial specialization of the member template is explicitly specialized for a given (implicit) specialization of the enclosing class template, the primary member template and its other partial specializations are still considered for this specialization
of the enclosing class template.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">struct</span> A <span class="co1">// enclosing class template</span>
<span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T2<span class="sy1">&gt;</span>
    <span class="kw1">struct</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>      <span class="co1">// primary member template</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T2<span class="sy1">&gt;</span>
    <span class="kw1">struct</span> B<span class="sy1">&lt;</span>T2<span class="sy2">*</span><span class="sy1">&gt;</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// partial specialization of member template</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T2<span class="sy1">&gt;</span>
<span class="kw1">struct</span> A<span class="sy1">&lt;</span><span class="kw4">short</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// full specialization of primary member template</span>
                       <span class="co1">// (will ignore the partial)</span>
 
A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy2">*</span><span class="sy1">&gt;</span> abcip<span class="sy4">;</span>  <span class="co1">// uses partial specialization T2=int</span>
A<span class="sy1">&lt;</span><span class="kw4">short</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy2">*</span><span class="sy1">&gt;</span> absip<span class="sy4">;</span> <span class="co1">// uses full specialization of the primary (ignores partial)</span>
A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> abci<span class="sy4">;</span>    <span class="co1">// uses primary</span></pre></div></div>
<p><br>
</p>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1315.html">CWG 1315</a>
</td>
<td> C++98
</td>
<td> template parameter could not be used in non-type<br>template arguments other than id-expressions
</td>
<td> expressions ok as long as deducible
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1495.html">CWG 1495</a>
</td>
<td> C++11
</td>
<td> the specification was unclear when involving parameter pack
</td>
<td> the specialization shall be more specialized
</td></tr></table>

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